Many different methods for the preparation of epoxides have been developed. One such method involves the epoxidation of an olefin in a liquid phase reaction using an organic hydroperoxide as the oxidizing agent and certain solubilized transition metal compounds as catalyst. Although this approach is practiced commercially and generally provides high selectivity to epoxide, it has at least two characteristics which tend to limit process flexibility and increase production costs. The use of an organic hydroperoxide results in the generation of a co-product alcohol derived from the reacted hydroperoxide during epoxidation; approximately 1 equivalent of the co-product is obtained for each equivalent of epoxide. If no market exists for the alcohol, the co-product must either be further reacted (incurring additional processing costs) so as to convert it back to the hydroperoxide oxidant or to another compound for which a commercial demand exists. Recovery of the soluble metallic catalyst used in such a process for reuse in subsequent runs is also problematic. It would therefore be highly desirable to develop an insoluble (heterogeneous) epoxidation catalyst which has high activity and selectivity when utilized with an oxidant such as hydrogen peroxide which does not form an organic co-product. Such a catalyst would ideally be readily recoverable in active form from an epoxidation reaction mixture by filtration or similar separation techniques or be capable of being utilized in the form of a fixed bed or the like.
Workers at the Universidad Politecnica de Valencia have recently reported the synthesis of a titanium silicoaluminate isomorphous to zeolite beta (see Camblor et al., J. Chem. Soc., Chem. Commun. pp. 589-590 (1992), Camblor et al., Zeolites 13, pp. 82-87 (1993) and ES 2037596 (published 6/16/93)). Such materials were found to catalyze the oxidation of alkanes to alcohols, ketones, and the like using hydrogen peroxide as the oxidant. This type of titanium silicoaluminate in unmodified (fully protonated) form is a poor catalyst for the production of epoxides from olefins, however.
Titanium silicalite, a zeolite of the ZSM family, is known to be an efficient epoxidation catalyst with hydrogen peroxide. In titanium silicalite, Ti atoms are located in vicariant positions in place of Si atoms in the crystalline framework. As noted in Romano et al., "Selective Oxidation with Ti-silicalite", La Chemica & L'Industria 72, 610-616, however, no Al atoms are present in titanium silicalite. The presence of Al atoms creates strongly acidic sites. As acids are known to catalyze epoxide ring-opening reactions, it was heretofore believed that a zeolite containing aluminum could not provide high yields of epoxide when employed as an olefin epoxidation catalyst since considerable amounts of by-products resulting from epoxide decomposition would result.